How Should I Prepare for a C# Interview at a Company Using Proprietary Software?

The word proprietary can cover anything from a normal internal application to an old, heavily customized system with little documentation. That isn't necessarily a problem, but it is worth asking what the day-to-day stack actually looks like, which .NET version they use, how much of the work is maintenance versus new development, and how new hires are trained. Also ask whether the interview includes debugging or working in an existing codebase.

Focus first on the technologies you already know are important. Review core C# and .NET concepts, object-oriented design, exception handling, collections, async programming, and common application architecture patterns. For SQL, practice writing joins, filtering, grouping, subqueries, and basic query optimization. A little review of Python, React, and VBA is useful, but don't spread yourself too thin.

They are unlikely to test you on an internal tool you could not have encountered before. More likely, they want to see how you reason when the requirements or code are unfamiliar. Practice explaining your assumptions, identifying what you would investigate, and describing how you would test a change. Reviewing application architecture, deployment practices, and possibly microservices would also be worthwhile if those are part of the role.

Ask the recruiter what format the interview will take. It could involve data structures and algorithms, discussing computer science fundamentals, debugging existing code, pair programming, or building a small application. Knowing the format is much more useful than guessing, and asking shows that you want to prepare properly.

“Proprietary” usually just means the tools and code were built internally, so you probably aren't expected to know them already. They may be looking for your ability to understand an unfamiliar codebase, make reasonable assumptions, and learn systems that aren't perfectly documented. Practice reading existing C# projects and explaining your thought process as you work.